See Rav Zevin's Moadim B'Halacha for Yom Kippur "Tzom He'asor." The achronim he cites all seem to focus on the last of the reasons offered by Rabbeinu Yonah in Shaarei Teshuva.
What are some nafka minas between the different answers of R"Y? One would be if you're not fasting (As R' Zevin mentions). To the third reason, there is no reason to eat a special meal. To the second reason, if someone is not fasting, perhaps they should eat a yom tov meal on Yom Kippur itself. To the first reason, they should perhaps eat on Erev Yom Kippur, since that is when everyone also is celebrating the kaparah.
As a note to the very last point, Rav Yoshe Ber zt"l said Sukkos is Zman Simchaseinu - the simcha is that of receiving the Kapparah on Yom Kippur, and we just wait a few days to celebrate.
Another Nafka mina would be should you eat the night before Erev Sukkos. The mashmaus of Rashi (Kesuvos 5a s.v. shechal lihyos) is that you do eat the night before Erev Yom Kippur, but no one has such a custom. To the first and third reasons, there is little rationale for eating on the night before Erev Yom Kippur. To the second reason, if we keep Erev Yom Kippur special because we can't eat on Yom Kippur itself, maybe there is some rationale to eat on Erev Yom Kippur eve.
On this second reason of R"Y, my Rebbi Rav Ahron zt"l explained that Yom Kippur should be a Yom Tov like Shavuos because Moshe returned with the second Luchos on Yom Kippur. So really Yom Kippur is a joyous celebration of receiving the Torah and that is why we have a seudah on Erev Yom Kippur.
Then there is a possibility that Erev Yom Kippur has some of the sanctity of Yom Kippur itself. One source is the din of tosefes Yom Kippur. The Gemara discusses this (Yoma 81b). "Bitisha lachodesh ba'erev, mi'erev ad erev..." The inui of Yom Kippur starts on the ninth of Tishrei. Rambam, however, only mentions tosefes for Yom Kippur (Shevisas Asor 1:6) and not for Shabbos or Yom Tov. We see, therefore, that Erev Yom Kippur already has some element of Yom Kippur. (And see Harirei Kedem I 47, you can interpret it like I am saying or you can interpret it differently.) Another possibility for this is in the Rambam, Hilchos Teshuva 2:7 regarding saying viduy before the seudas hamafsekes. Rav Yoshe Ber said (see Machzor Mesoras Harav, p. xix) that "Erev Yom Kippur is appended to Yom Kippur in respect to the kedushas hayom." We could suggest, though it is not totally compelling, that if the ninth is special, its uniqueness begins the night before - that is, the night before the ninth of Tishrei.
According to this, let me depart for a moment from my normal rigid seriousness. The mekubalim say Yom Hakippurim means "the day which is like Purim." Now Yom Kippur is from Har Sinai and Purim was not until after chrurban bayis rishon? Using the previous idea - that Yom Kippur is really 2 days (or 1+ days), we can compare it to Purim. See Rambam Taanios 5:5 that 13 Adar, commonly known as Taanis Esther, is midivrei Kabbalah, because it is from the pasuk in the megillah "divrei hatzomos vezaakasam." (Note: this Halacha in the Rambam needs a lot of explanation. The simple reading says that we fast the 4 fasts as minhag, but Taanis Esther is midivrei Kabbalah. See Magid Mishna, that Taanis Esther is not mentioned in the Gemara. But the 4 fasts are, so it seems that the fast mentioned in the Gemara would have a higher status? One answer is that Taanis Esther is midivrei Kabalah. And if you want to suggest the 4 fasts are also divrei kabbalah because of the passuk in Zecharya tzom harevii vetzom hachamishi - please comment, because I don't think that is a compelling question. A simple but elegant explanation is that of Rav Zelmeleh, the brother of Rav Chaim Volozhiner, that remove the vav from "u'v'yud-gimmel baAdar." Thus this entire Halacha is only discussing Taanis Esther and none of the 4 fasts.) Now, my brother told me that our Rebbi Rav Moshe shlit"a and his brother Rav Yosef shlit"a gave the same shiur with opposite conclusions. In 2 sentences, why is Taanis Esther called Yemai Hatzomos - that would include Purim as a tzom? They both answered that Purim and Taanis Esther are one combined holiday. However, Rav Moshe said that Taanis Esther is a happy fast, and Rav Yosef answered that Purim is a sad holiday. regardless of that dispute, having a holiday where one day you fast and one day you eat, would be like Purim, and thus Yom Kippurim - that we eat on the ninth and fast on the tenth - is like Purim, where we fast on the 13th and eat on the 14th.